The long-term objective of this application is to understand the genetic and hormonal mechanisms controlling gonad development in vertebrates. Primary sex determination in most vertebrates results in the development of a single gonad, ovary or testis, from a bipotential primordium, whose developmental fate is controlled by genetic or environmental mechanisms. In some instances a dysgenic gonad may form as a result of congenital or environmental perturbation, occurring either early or late during development, respectively. The frequency of gonadal dysgenesis in human births is 1/15,000, and more extreme is the plasticity observed in fish species capable of whole organism (including gonad) sex-reversal in later stages of development. This application makes use of a synchronous self-fertilizing hermaphroditic fish, Kryptolebias marmoratus (Kmar), whose unique form of reproduction involves a mixed gonad structure referred to as an ovotestis (testis and ovaries in the same place). The ovotestis is capable of normal gametogenesis and fertilization within a common lumen. Most Kmar are configured this way and can easily be self-crossed through several generations to genetic isogeny. The main aim of this application is to perform a grand childless genetic screen in Kmar for mutations involved in ovotestis development. The hypothesis is that mutants derived from this screen will be sterile by disrupting ovary or testis formation within the mixed ovotestis environment. By comparison of mutants to wild-type individuals of identical clonal decent, a default mechanism is hypothesized that is applicable to understanding the predominant bipotential terminal-mode of gonad organogenesis in vertebrates. Several phenotypic classes are predicted from this genetic screen, including a mutant ovotestis (forms only ovary or testis) resistant to hormonal sex-reversal when exposed to estrogen. This type of genetic screen is useful for understanding hormonal regulation of gonad development, and may only be detectable in the context of an otherwise mixed gonadal primordium, the ovotestis, found only in this species.